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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3726
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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I'm working on a 1975 mercury 500 50 hp outboard serial

Customer Question

I'm working on a 1975 mercury 500 50 hp outboard serial #4143669 with no spark no start. I've removed the tilt switch and shut off circuits from the blue terminal of the switch box. I've checked the resistance between the blue and red circuits and have 430 ohms. Red to white circuit 439 ohms. Blue to white circuit 10 ohms. Using a Fluke 77 on ac voltage red to white while cranking and get 98 volts and around 10 volts on the blue and white wire while cranking. These were both measured at the switch box. I have 0 volts at the green wire at the switch box while cranking. The coil is new. All connections at the switch box and coil are new and in great shape. Not sure how to test the switch box or what to do next. Thank you in advance.
Submitted: 1 year ago.
Category: Boat
Expert:  Charles replied 1 year ago.

Hello, my name is***** will assist you.

since you have voltage going to the switch box and nothing coming out, it is the switchbox that is the failed part. You have tested it correctly already.

Customer: replied 1 year ago.
The resistance readings are close enough then? I thought the voltage needed to be 280 and 39 volts? The switch box part# *****(###) ###-####3 fyi.
Expert:  Charles replied 1 year ago.

You have 2 blue wires and 2 red wires correct?

Customer: replied 1 year ago.
No one of each. Its a type 2 ignition.
Expert:  Charles replied 1 year ago.

Ok, I have it. disconnect the wires and try the test with them unloaded.

Customer: replied 1 year ago.
For voltage? Will do first thing in the morning.
Expert:  Charles replied 1 year ago.

Yes the voltage test. Same as you have done them today.

Customer: replied 1 year ago.
Red to white while cranking I have 96 volts a.c. and on the blue to white it was 9.6v a.c..
Expert:  Charles replied 1 year ago.

It is low, but may still work. Spark would be slightly weak when running.

Customer: replied 1 year ago.
Still believe its a switch box? Or should I be testing other components?
Expert:  Charles replied 1 year ago.

Yes, even though the voltage is on the lower side from the stator, there is 0 coming out to the green wires.

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